18_Ch 17 College Physics ProblemCH17 Current and Resistance

18_Ch 17 College Physics ProblemCH17 Current and Resistance...

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86 CHAPTER 17 17.41 The total power converted by the clocks is ( )( ) 68 2.50 W 270 10 6.75 10 W = × P ( and the energy used in one hour is )( ) 81 10 J × 2 6.75 3 600 s 2.43 Et =⋅= × = P The energy input required from the coal is 12 2.43 10 J 9.72 10 0.250 coal E E efficiency × == = 12 J × The required mass of coal is thus 12 6 9.72 10 J 2.95 33.0 10 J kg coal E m heat of combustion × × () 5 10 kg = × or 5 3 1 metric ton 2.95 295 me m  =   tric tons 17.42 (a) ( ) ( ) 4 40.0 W 14.0 d 24.0 h d 1.34 10 Wh 13.4 kWh = × = P ( ) ( ) 13.4 kWh $0.120 kWh $1.61 cost E rate =⋅ = = (b) ( ) 2 0.970 kW 3.00 min 1 h 60 min 4.85 10 kWh = ×
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